This application is a continuation-in-part of prior U.S. patent application Ser. No. 879,970, filed June 30, 1986, now abandoned.
This invention relates to monitoring the axial loading upon the rotor assembly of a rotating machine and automatically adjusting the load to maintain the axial forces in a balanced condition while the machine is operating.
The term "rotating machine" as herein used shall include centrifugal compressors, pumps, turbines, turbo-expanders, and the like which contain a rotor assembly that is subjected to varying axial loads. Although rotor assemblies of this type generally contain a pair of opposed thrust bearings that are designed to resist axial forces, it has been found that the thrust bearings alone cannot completely absorb the entire load acting upon the structure without incurring high power losses and unbalanced rotor conditions.
As described in U.S. Pat. Nos. 3,828,160 and 3,895,689, which were issued to Swearingen, most thrust bearings typically involve a stationary component secured in the machine casing and a rotating or moving component that is secured to the rotor shaft. A lubricant or oil is introduced into the bearing gap separating the stationary and moving bearing components. As the machine approaches operational speeds, the oil forms a hydrodynamic wedge that resists to some extent axial shifting of the rotor and prevents the bearing components from rubbing under normal operating conditions.
As further disclosed by Swearingen in the above noted patents, a rotor balancing system is provided that offsets the axial thrust acting on the rotor of a centrifugal compressor to prevent damage to the bearings. The system includes a chamber formed in the machine casing that is closed against the impeller wheel by means of labyrinth seals. The chamber is connected to the suction side of the compressor and the compressed fluid is allowed to bleed back through the chamber to provide a resistive force against the impeller. This in turn prevents unwanted rotor shifting. A valve is positioned in the suction line of the Swearingen system to control the flow of fluid through the chamber. In addition, Swearingen controls the positioning of his valve by sensing the oil pressure in opposing thrust bearing used to support the rotor structure. Pressure taps are placed in the bearings and the pressure in each bearing is sensed. In one form of his invention, Swearingen connects his pressure taps to a controller which automatically adjusts the balancing chamber control valve setting.
Chang et al. in U.S. Pat. No. 4,472,107 describes an improvement of the Swearingen balancing system. In Chang, the pressure tap lines are connected over a piston. The piston, in turn, is connected either electrically, hydraulically or mechanically to a relatively complex valve arrangement which, as in Swearingen, controls the flow of working fluids through a balancing chamber. The pressure in the chamber is controlled so that the axial forces acting on the rotor are substantially balanced.
Both the Chang and the Swearingen systems are dependent upon bearing pressures to determine the position of the rotor, and accordingly, these systems are susceptible to any and all problems associated with thrust bearings. Excessive heating of the bearings will produce errors in pressure readings and thus adversely effect balancing conditions. Blockage of the lubricant flow passages can again produce erroneous pressure readings as can misalignment in the bearing components. Both the Chang and the Swearingen systems are capable of working well at operational speeds, however, little or no useful thrust information can be acquired by the balancing system at start up or at low operating speeds.